In some types of wireless communication systems, such as Wideband Code Division Multiple Access (WCDMA) systems, interference suppression (IS) is used in order to achieve better performance in terms of peak data rates, coverage, system throughput, and system capacity. Examples of such interference suppression receivers that are commonly used include the GRAKE+ receiver, the Frequency Domain Equalization (FDE) receiver, and the Frequency Domain Pre-Equalization (FDPE) receiver.
The user terminals, also called user equipments (UEs), in WCDMA systems are subject to fast inner loop power control for uplink (UL) transmissions. The power control loops steer the powers of the user terminals so as to achieve desired signal-to-interference-plus-noise (SINR) targets for the user terminals. The power control loops for all user terminals are coupled nonlinearly through the SINR measurement (the so called “party effect”). It is known that the power control loops remain stable as long as the uplink is operated below its pole capacity. However, when the system is close to the pole capacity, stability is reduced and power rushes can occur. In a power rush, the user terminals increase their uplink transmission power, which causes additional interference. The base station tries to counteract the increase in interference by instructing the user terminals to further increase their transmit powers in order to meet the target SINR, which further increases the interference. Due to the positive feedback loop, the system may not be able to meet the target SINR and a power rush may cause a system outage.
Instability is a significant problem in WCDMA systems because the power control loops are very fast and capable of stepping up the transmission power of the user terminals at a rate of 1500 dBs/second. Mechanisms to prevent such power rushes are therefore necessary where the uplink in a WCDMA system is to be operated close to the pole capacity.
Stability monitoring based on estimates of the load as seen at the base station has been used in the past to prevent power rushes. Consecutive estimates of the load are determined at a rate at least as fast as the power control loop. Functions of the load estimate, such as the rate of change of the load and the absolute level of the load, are then compared to corresponding thresholds and a power rush is declared if the functions exceed the thresholds.
The stability monitoring techniques used in the past were not developed with interference suppression receivers in mind. Therefore, the load estimates used in conventional receivers will be higher than the effective load experienced by a base station using interference suppression receivers. Also, with interference suppression receivers, the power control loops are closed after interference suppression processing. Known techniques for measuring the load after interference suppression are either too computationally complex, or not suited for stability monitoring. For example, in U.S. Patent Application 61/419,447 filed Dec. 3, 2010, the rise over thermal (RoT) after interference suppression is used for fast congestion control (FCC) purposes. However, strictly speaking, the RoT is related to coverage and not cell stability. Finally, the load estimates for interference suppression receivers should account for neighbor cell interference. There are currently no efficient methods available for measuring the neighbor cell interference.
Additionally, interference suppression receivers are more sensitive to the overall interference on the uplink as compared to conventional receivers. Therefore, the power control loops for interference suppression receivers are more prone to power rushes than the power control loops of conventional receivers. Consequently, the need for stability monitoring will become increasingly important when interference suppression receivers are introduced in the uplink in WCDMA systems.